Epoxy resins currently used in the manufacture of high performance composites all tend to suffer from loss of tack, drape, and usability under conditions of ambient handling and storage due to chemical advancement, i.e., partial curing, of the resin. These epoxy compositions typically consist of a poly-O-glycidyl ether and/or poly-N-glycidyl amine, and an aromatic amine curing agent such as 3,3'-diaminodiphenyl sulfone, 4,4'-diaminodiphenyl sulfone or fluorenedianiline, and various other additives. These compositions are typically used in a hot melt process to form a prepreg. Hot melt processing may involve mixing, filming and prepregging at temperatures between 80.degree. C. and 130.degree. C. After hot melt processing these standard curing agents can react with the polyepoxide component even at low temperatures, especially in the presence of moisture. These reactions lead to limited out-time under conditions of ambient storage.
Out-time, which is a function of temperature and environment, is important in the construction of large and/or intricate parts where lay-up procedures can often exceed one month. For example, humidity is known to catalyze reactions in epoxy resin compositions at low temperatures. Systems that do not advance during the manufacture of parts provide better control of resin flow and therefore produce more homogeneous composites with better mechanical performance. Resistance to advancement also provides better product quality control.
Out-time is defined as the period over which tack, drape and usability are preserved by the resin composition (and products containing the resin composition such as prepreg) under defined environmental conditions. Tack is a subjective estimate of the ability of two resin-containing structures (such as prepreg) to stick together so that alignment is not easily disturbed. Should realignment be necessary, however, no damage is suffered to the objects upon removal and realignment. An example of such an alignment process is found in the lay-up procedure for carbon fiber-reinforced composite structures. Drape is the ability of a resin-containing structure (such as prepreg) to be bent and shaped without fracture or rebound of the uncured resin. Maintenance of tack and drape requires that resin advancement be kept to a minimum so that the glass transition of the resin does not exceed a critical level, which is typically about 20.degree. C. "Usability" indicates that cured resin and composite properties are not deleteriously affected by conditions encountered prior to cure.
Epoxy resin compositions in which an aromatic amine curing agent becomes dissolved during mixing, filming and prepreg manufacture are described, for example, in U.S. Pat. No. 4,940,740. The preferred curing agents are 2,4'-, 4,4'- and 3,3'-diaminodiphenylmethane, 2,4'-, 4,4',- and 3,3'-diaminodiphenyl sulfone, 2,2-bis(4-aminophenyl)propane and 2,2-bis(3-aminophenyl)propane. U.S. Pat. Nos. 4,330,659 and 4,500,582 disclose the use of the reaction product of diaminodiphenyl sulfone compounds and glycidyl ethers of polyhydric phenols as a hardener for epoxy resins. Storage stability of these resin compositions is poor under ambient, humid conditions. Resin compositions described as having improved storage stability are disclosed in U.S. Pat. No. 4,933,392, where compositions containing dicyandiamide (DICY) or polycarboxylic acid hydrazide are reported to be stable at 40.degree. C. for up to 45 weeks. However, these curatives do not provide the thermal and mechanical properties attainable with aromatic amine curing agents, and at levels .ltoreq.10 parts DICY per hundred parts polyepoxide tend to cause resin advancement problems during hot melt processing.
Aromatic diamines such as those disclosed in the present invention have been disclosed previously, although not their use as curing agents for epoxy resins. For example, U.S. Pat. No. 2,684,372 discloses the preparation of 2,7-diaminothioxanthone dioxide. JP 62-028792 discloses the preparation of 3,7-diaminodibenzothiophene-5,5-dioxide, which is said to be useful in preparing aromatic polyamides.